In a politically polarized nation, what constitutes the middle ground?

The answer is not as simple as it might seem. We are in a time in which there are both rising expressions of independence from the two major parties by many Americans and elections in which the red-blue divisions are increasingly stark.

Party identification tells one part of it, the story of a country moving away from allegiance to the major political parties. A decade ago, about one-third of Americans described themselves as independents, according to Gallup surveys. Today that’s grown to four in 10 or more. In some states that allow registration by party, the biggest increases have been among those who decline to identify with either the Republicans or Democrats.

Voting behavior tells a different story. In recent elections, at least nine of every 10 people who identify themselves as Republicans or Democrats — or who say they are independents but lean toward one party or the other — vote for the candidate of their party down the ballot. In 2012, only about 11 percent of voters said they cast split tickets. The percentage of true independents may be only about 10 percent of the electorate.

The trend toward polarized politics is well documented. From the most recent studies by the Pew Research Center to a sizeable body of continuing work by political scientists, it’s clear that partisanship drives a considerable portion of the electorate. The gap between those on the left and right — especially among the most politically engaged citizens — is deeper and more passionately expressed that it was in the past.

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About a fifth of the population is now either consistently conservative or consistently liberal, according to Pew’s analysis. Add to that those citizens who are generally conservative or generally liberal and that accounts for, roughly, an additional 40 percent of the population. That leaves about four in 10 somewhere in the ideological middle. According to Pew, that middle ground has shrunk over the past decade or so, when it accounted for half the population.

Those in the middle are often assumed to be moderate in their political outlook. If that’s the measure, they too constitute a smaller share of the electorate than they once did. Until 2009, according to Gallup’s historical tables, moderates were the largest group in the electorate — more than four in 10. Last year, 34 percent of Americans identified themselves as moderate, the lowest found by Gallup in its polls.

Today a plurality of people describe themselves as conservatives — but the group that has risen most rapidly in the past few years are those who call themselves liberals.

Independents are still more likely to call themselves moderates than as liberals or conservatives. What Gallup has seen in recent years is that more and more independents describe their ideology as conservative.

The reason for that, according to Gallup’s analysis of the numbers, is that people who once called themselves Republicans now say they’re independents. Their party identification has changed but not necessarily their ideology.

Still another factor that complicates the picture is the fact that people who may be classified as part of the political middle aren’t necessarily in the middle of the electorate and doesn’t mean they really are moderate in their views.

The Pew study in fact found something quite different. People who didn’t fall into the polarized extremes sometimes hold views similar to those who are. They’re just not consistent about it. “Being in the center of the ideological spectrum means only that a person has a mix of liberal and conservative values, not that they take moderate positions on all issues,” according to the Pew analysis.

One consistent finding is that those who now constitute the middle are less active politically than those on the left or right. “The voters in the middle tend to be much less engaged in politics than those near the poles — less interested, less attentive, less knowledgeable and less active,” said Alan Abramowitz, a political science professor at Emory University. “So the more active and knowledgeable the set of voters, the more polarized they tend to be.”

The Pew study looked at the electorate in another way, grouping people into different categories based on a variety of measures. This typology, the latest in a series dating to 1987, described eight distinct groups among the population. Seven of the groups are politically engaged. The other is on the sidelines — not even registered to vote.

Three of the seven politically engaged groups are the partisan anchors for either the Democrats or the Republicans. “Steadfast Conservatives” and “Business Conservatives” are loyal to the Republican Party and “Solid Liberals” are attached to the Democratic Party. Together they make up 36 percent of the population, 43 percent of registered voters and 57 percent of the people who are politically engaged.

Among those not at the polarized wings of the electorate are three groups that lean toward the Democrats — “Hard-Pressed Skeptics,” “Next Generation Left” and “Faith and Family Left” — and one that aligns with the Republicans — “Young Outsiders.” These four groups make up 54 percent of the population but only 43 percent of politically engaged people.

Notably they are more difficult to categorize in their political behavior. As the Pew study put it, they are “less partisan, less predictable and have little in common with each other or the groups at either end of the political spectrum. The one thing they do share is that they are less engaged politically than the groups on the right or left.”

However disparate, however disengaged and whatever its size, the middle of the electorate cannot be ignored by either party. The shifting sentiments of these voters have caused big swings in elections over the past decade. In 2006, independents swung one way and helped Democrats take control of the House. In 2010, they went the opposite way and gave Republicans control of the House.

Gary Jacobson, a political science professor at the University of California at San Diego, describes the political middle this way in an e-mail message: “It does not form a potentially coherent coalition around which some political entrepreneur might build a centrist party,” he wrote. “People in it are more susceptible to short-term political tides (because they are less partisan and ideological) and thus help to swing elections.”